Preheat oven to 375°F. Warm 3 Tbsp. vegetable oil over medium-high heat. Add 1 sliced onion and 1 sliced red bell pepper; sauté until softened, 5 minutes. Add 1 minced garlic clove, 2 sliced zucchini and 2 sliced yellow squash; cook, stirring, until vegetables are tender and begin to brown, 10 minutes. Season with salt and pepper.
Mist an 8-inch, square baking dish with cooking spray. Cut a 17-ounce tube of prepared polenta into 18 slices. Spread 1/2 cup jarred marinara sauce in bottom of dish. Top with 9 slices of polenta, then half of vegetable mixture. Sprinkle 1 cup shredded Italian cheese blend over vegetables. Repeat layering, beginning with sauce, followed by polenta, then vegetables and then cheese.
Bake until heated through, cheese is melted and sauce bubbles, 35 minutes. Let rest for 10 minutes before serving. Serve hot.